It is known that in image signal digital transmission systems it is desired to reduce as much as possible the quantity of information to be transmitted for a given image quality and, to this end, several coding systems allowing image redundancy reduction have been investigated.
The most promising technique at low bit rates has been considered so far to be the so-called hybrid coding, which is a combination of transform coding and of differential coding and exploits both spatial and temporal image correlations. However, it appears that the upper performance limits of this technique have been substantially reached, and hence studies are being performed to detect new techniques allowing enhancement of the quality of low bit rate coded images, and, more particularly, techniques based on a certain degree of knowledge of the image to be coded and/or transmitted.
In the particular case of visual telephony, the coding process can take advantage of the fact that the background remains practically unchanged while the speaker figure changes. Therefore, it is possible to transmit and store in the receiver once coding for all the information relevant to the background, while at each frame only the information relevant to the speaker figure will be coded and/or transmitted.
A method of processing videotelephone images allowing the separation of the speaker figure from the background has been described by L. Westberg in the paper entitled "Pyramid based object-background segmentation of dynamic scenes", Picture Coding Symposium 1988, Torino, 12-14 September 1988. According to this known method, the image is split into blocks of elements, the differences between corresponding blocks in successive frames are determined, such differences are compared with a threshold and the blocks are classified as belonging to the background or to the object depending on whether the difference is or is not below the threshold. The operations are repeated with blocks of smaller and smaller size, yet composed of a two-dimensional matrix of elements. The blocks belonging to the object are then supplied to downstream processing devices, e.g. coding units.
This method allows detection of a region presumably comprising the object border, which region consists of the blocks which were classified neither as background nor as object; operating on blocks of elements does not allow detection of the object contour line.